140 Prof. Allman on the Hydroid Zoophytes. 



(the ectothequc) ; and between these two is a third membrane, 

 which has been hitherto overlooked, and which is the true 

 representative of the umbrella of a Medusa. It is on the 

 summit of this middle sac, or mesotheque, as it may be appro- 

 priately called, and not on that of the ectothequc, as I formerly 

 believed, that the short tentacles which crown the sporosac 

 originate ; they surround an orifice in the mesotheque, and pass 

 out through a corresponding orifice in the ectothequc. The 

 walls of the mesotheque are thickened round the orifice, and 

 here apparently contain a circular canal, as indicated by the 

 presence of coloured granules : if radiating canals really exist, 

 it is in this membrane that they are developed. 



The tentacles possess, like the marginal tentacles of a true 

 Medusa, considerable contractility. They may frequently be 

 seen of very different lengths in different sporosacs of the same 

 specimen ; and this, which is really the result of different degrees 

 of contraction, may be easily taken for different degrees of de- 

 velopment, the tentacles being especially sluggish in the act of 

 extension or contraction. Their length when fully extended, in 

 the female sporosac, will equal about half the diameter of the 

 sporosac, while under external irritation they will slowly contract 

 to a third of their original length, and will then show themselves 

 as a little stellate crown on the summit of the sporosac. They 

 vary in number ; I have counted in the female sporosac from 8 

 to 16 or 20. They are composed of ectoderm and endoderm, 

 the ectoderm containing thread-cells, and the endoderm present- 

 ing the usual septate appearance. They are less numerous and 

 less developed in the male than in the female. 



That the bodies now described belong to the class of sporosacs 

 rather than Medusa?, must, I think, be admitted. In all essential 

 points they agree with the sporosacs of Tubularia indivisa, in 

 which a mesotheque is fully developed*; and it should also be 

 borne in mind that when Medusa? are produced in the Laomedete 

 and Campanularia? they belong to a different type, the generative 

 products being developed upon the course of the radiating canals, 

 and not, as in these medusiform sporosacs, in the walls of the 

 manubrium. 



They are thus of no little interest in the morphology of the 

 zoophytes ; and it will be found convenient to speak of them by 

 a special name. Their singular resemblance, especially when 

 their tentacles are contracted, to a poppy-capsule with its sessile 

 stellate stigma, will instantly strike us, and has suggested to me 

 the name of meco?iidiaf, by which I propose to designate them. 



* See " Notes on Hydroid Zoophytes " in a previous Number of the 

 " Annals.' 

 t From fit]Koov, a poppy. 



